With the recent enlargement of the field of applications of light-emitting diodes (hereinafter also referred to as “LEDs”), light-emitting diodes which are more highly reliable and capable of emitting high-brightness light for a long period of time have been demanded. A light-emitting diode is constituted of external electrodes, a LED chip, a package comprising an aperture having a bottom surface and sidewalls for efficiently taking out the light from the LED chip, and a sealing member. On the package bottom, the external electrodes are formed by monolithic molding, and there is formed a high-reflectivity noble metal film for satisfactorily taking out the light outwardly. Since, on the other hand, the package is generally made of an organic resin, continued irradiation of the sidewalls with light causes problems, for example a decrease in adhesion to the sealing member due to deterioration of the package resin, a decrease in reflectivity due to discoloration of the package resin and the resulting decrease in brightness of the LED.
Therefore, a light-shielding paste for protecting the package resin has been demanded for the purpose of avoiding the direct light irradiation of the LED package and/or for the purpose of preventing the decrease in reflectivity due to discoloration of the package resin.
In the prior art, a technology applicable to adhesive sheets, finished cloths and the like has been proposed which utilizes a curable composition comprising a organosilicon compound, which is an addition reaction-curable (hydrosilylated) silicone having epoxy group- and alkoxy group-bound silicon atoms, and an organoaluminum compound and showing good adhesive properties (Japanese Kokai Publication Hei-08-183934, Japanese Kokai Publication Hei-05-140459).
From the optical characteristics viewpoint, however, the materials obtained by this technology are not suited for use as the light-shielding resins for light-emitting diodes.
Further, a technology of improving the adhesive properties of addition reaction-curable (hydrosilylated) liquid compositions by adding an epoxy resin and an aluminum compound has been proposed (Japanese Patent No. 3354973). However, there is no disclosure therein about such light-shielding resins for light-emitting diodes.
Further, it has been disclosed that when a condensation reaction-curable silicone is used as a sealing material for building joints, the addition of a borate ester thereto enables manifestation of good adhesion to adherends even when the joints are shallow (Japanese Kokai Publication Sho-59-155483). However, this system is a room temperature curing one, and there is no disclosure either about effect manifestation upon heating for curing or about effect manifestation in the case of application to addition reaction type systems differing therefrom in mode of reaction.
On the other hand, it is known that curing products obtained from a curable composition comprising an organic compound having, within the molecule, at least two carbon-carbon double bonds reactive with a SiH group, a compound containing at least two SiH groups within the molecule, and a hydrosilylation catalyst are applicable as sealants for light-emitting diodes (Japanese Kokai Publication 2002-314140). However, there is no disclosure about the adhesion after light irradiation, which is required of light-shielding resins for light-emitting diodes.
Meanwhile, the light-shielding layers can indeed be formed by curing a curable composition but there is a problem in that when the curable composition is high in flow leveling property, it flows onto the package bottom during curing, leading to failure to obtain light-shielding layers having a desired thickness. When the curable composition is high in flow leveling property, another problem arises, namely the bottom is stained and, for example, it becomes impossible to gain electric access to lead frame members disposed on the bottom.
Furthermore, it is also a problem that no effective method is available to coat the LED package aperture sidewalls alone. By saying “to coat the LED package aperture sidewalls alone” herein, it is meant that even the case where the light-shielding paste of the invention flows out onto those portions of the bottom which are in the vicinity of the sidewalls and is cured in that state without producing any adverse effect on the performance characteristics of the final product light-emitting diodes is included as well within the meaning thereof.